Recently, considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are being rapidly depleted, the supply of hydrogen remains virtually unlimited. Hydrogen is a relatively low cost fuel and has the highest density of energy per unit weight of any chemical fuel. Furthermore, hydrogen is essentially non-polluting since the main by-product of burning hydrogen is water. However, while hydrogen has enormous potential as a fuel, a major drawback in its utilization, particularly in automotive applications, has been the lack of an acceptable on-board hydrogen storage medium.
Hydrogen storage in a solid matrix has become the focus of intense research because it is considered to be the only viable option for meeting performance targets set for such automotive applications. Although there are many technical targets and design criteria surrounding a viable hydrogen storage system, four important targets appear to be system volume and weight, discharging and charging rates, thermal management associated with charging, and dormant system over-pressurization.
It is common for many materials to release copious amounts of heat during charging. In this regard, it is extremely difficult, if not impossible, to fill an on-board storage system with hydrogen in a short period of time because of inadequate thermal management unless a complicated heat exchanger system is integrated into the on-board filling operation. Such a problem can be resolved with off-board refilling of an easily exchangeable canister. It is also common for many materials to release hydrogen uncontrollably during minimal dormant heating. In this regard, it is very difficult, if not impossible, to prevent hydrogen from being vented to the environment to circumvent over-pressurization of the storage system during dormant heating unless a complex on-board “hydrogen on demand” type system is integrated into the on-board storage system. However, such a problem can be resolved with a material that only releases hydrogen at temperatures above some minimum level.
One of the more promising classes of hydrogen storage materials being studied is the complex hydrides, such as NaAlH4. The dehydrogenation of NaAlH4 is thermodynamically favorable, but it is kinetically slow and takes place at temperatures well above 200° C. The dehydrogenation temperature and the kinetics of dehydrogenation can be markedly improved by the addition of a dopant or co-dopants, such as titanium chloride. Graphitic structures, such as fullerenes, diverse graphites and even carbon nanotubes, can also play an important role in improving the kinetics of dehydrogenation and reversibility of certain complex metal hydrides. Rehydrogenation of the NaAlH4 system is typically carried out at greater than 100° C. and greater than 1,000 psig to achieve reasonable kinetics and conversions. While the NaAlH4 system is attractive for hydrogen storage because it contains a relatively high concentration of useful hydrogen, its modest weight percent of hydrogen storage capacity is a major drawback toward commercial vehicular applications.
Other complex hydrides, such as LiAlH4, have much better hydrogen storage capacities. However, some complex hydrides, including LiAlH4, do not exhibit any reversibility under conditions that cause the NaAlH4 system to easily rehydrogenate. Good reversibility and fast kinetics are both needed to enable hydrogen storage materials to be capable of repeated absorption-desorption cycles without significant loss of hydrogen storage capacity and at reasonable charge and discharge rates.
Therefore, a need exists to develop materials and methods for reversible hydrogen storage in complex hydrides. Transportation and stationary applications may become more feasible when such approaches are utilized in the development of reversible hydrogen storage materials.